The present disclosure relates to a developing device for developing an electrostatic latent image formed electro photographically and an image forming apparatus provided with the developing device.
In recent years, a developing device employing a touch-down type (also referred to as a hybrid type) has been well known. In this type, a two-component developer including at least a carrier and a toner is supported on a surface of a developer supporting rotator, such that a magnetic brush is formed by the carrier. The toner supplied from the magnetic brush to a toner supporting rotator forms a toner layer on a surface of the toner supporting rotator, such that the toner flies from the toner layer to an image carrier. An electrostatic latent image on a surface of the image carrier is developed into a toner image.
In the developing device of touch-down type, the toner in the toner layer formed on the surface of the toner supporting rotator may remain on the surface of the toner supporting rotator without being used for developing. In such a case, degraded residual toner may cause reduction in density of an image, and long-neglected undeveloped toner may cause degraded image quality and developing defect such as developing ghost.
In order to suppress generation of the above-mentioned developing defect, it is important to collect (strip) the undeveloped toner remaining on the surface of the toner supporting rotator toward the developer supporting rotator between development operations performed in series. As a method for stripping the undeveloped toner, a developing device has been known, in which magnetic members are arranged to be opposite to each other in polarity between the toner supporting rotator and the developer supporting rotator. In this configuration, magnetic members in the developer supporting rotator are supported such that a peak of a magnetic force is positioned more upstream in a rotational direction of the toner supporting rotator than a line connecting rotational centers of the toner supporting rotator and the developer supporting rotator.
In the abovementioned developing device, a magnetic brush is formed between the toner supporting rotator and the developer supporting rotator, while the magnetic brush inclines downstream in a rotational direction of the developer supporting rotator. In this manner, a force of constraint by the magnetic brush can maintain stripping of toner while preventing the developer from accumulating between the two rotators and from escaping from the magnetic force of constraint.
In this connection, it is necessary that the length of the magnetic brush formed on the surface of the developer supporting rotator is substantially the same as the length of the toner supporting rotator in a direction of a rotational axis (longitudinal direction), in order to securely strip all the toner remaining on the surface of the toner supporting rotator. If the length of the magnetic brush is too much greater than the length of the toner supporting rotator, it is likely that the carrier scatters upward inside the housing from end portions of the magnetic brush in the longitudinal direction and a bias voltage applied between the toner supporting rotator and the developer supporting rotator leaks to a flange portion of the toner supporting rotator, which is composed of an electrically conductive material for application of bias.
On the contrary, if the length of the magnetic brush is too much smaller than the length of the toner supporting rotator, it is likely that stripping of the residual toner by the magnetic brush is insufficient and the toner remaining on the surface of the toner supporting rotator scatters about and generates smudges in an end portion of an image.
In addition, when the length of the magnetic brush is too much smaller than the length of the toner supporting rotator, it is likely that gaps occur at both longitudinal end portions of the toner supporting rotator and the toner and carrier agitated by an agitation roller inside the developing device scatter through the gaps.